Such a system is known from DE 197 20 205 A1. Since the purification of nitrogen oxides with the addition of a reducing agent is also known as selective catalytic reduction, the system is also referred to as SCR (selective catalytic reduction) system. By means of the reducing agent, the nitrogen oxides NOx (primarily nitrogen monoxide NO and nitrogen dioxide NO2) contained in the waste gas are catalytically converted into nitrogen and water at a temperature of 200 to 450° C.
The known SCR system has a first reactor with two heat-accumulator chambers filled with heat-accumulator materials. The raw gas is preheated in the one heat-accumulator chamber, reheated in the combustion chamber and then, after the reducing agent has been added, supplied to a second reactor with the reduction catalyst. The heat-accumulator material in the second heat-accumulator chamber of the first reactor is heated by means of the denitrified hot clean gas exiting the reactor. After that, switching takes place, i. e. raw gas is supplied to the second heat-accumulator chamber in reverse flow and denitrified hot clean gas from the SCR reactor to the first heat-accumulator chamber etc.
Due to the separate SCR reactor and the addition of the reducing agent to the waste gas exiting the combustion chamber, the known system inter alia requires a significant expenditure for equipment. In addition, when the heat-accumulator chamber to which raw gas has been supplied is switched over to clean gas, the entire raw gas volume existing in this chamber is flushed into the clean gas channel, as a result of which the denitrification efficiency will be noticeably reduced.
The most frequently used SCR catalysts contain titanium dioxide as the main component (carrier material). Vanadium pentoxide as well as tungsten and, if necessary, molybdenum compounds are secondary components. In JP 76-68907, for example, a catalyst is described which consists of V and Nb components as active components on a TiO2 carrier. A catalyst described in DE 38 21 480 contains TiO2, V, Mo and/or W and Zn. In DE 26 17 744, tin is also cited as an optional active component. However, a plurality of different catalyst compositions were also described, for example Fe on oxidic carriers (EP 0 667 181 A1), various active components on zeolite carriers, for example Ce (WO95/17949), Cu (DE 44 13 359), Ag and Pt (EP 0 682 975 A1) or ordinary metal oxide catalysts, for example spinel ZnAl2O4 (EP 0 676 232 A1). In addition, a dioxin and furan depletion is successful in case of SCR catalysts (WO91/04780).
DE 199 05 733 B4 discloses a regenerative catalytic post-combustion system using 2n+1 pairs of heat-accumulator chambers which are each connected by means of a switching chamber, with n being an integer greater than 0. For the purpose of preheating, the raw gas to be purified is alternately supplied to the heated heat-accumulator material of n pairs of heat-accumulator chambers, and the denitrified hot clean gas is supplied to the heat-accumulator material of further n pairs of heat-accumulator chambers in reverse flow. In this process, one pair of heat-accumulator chambers is purged. The reduction of the nitrogen oxides with the reducing agent takes place on a catalyst material arranged in the heat-accumulator chambers on the side of the combustion chamber. Here, the reducing agent is added directly to the raw gas.
This arrangement is disadvantageous if the raw gas contains sulphur components, which, together with the reducing agent added, form solid deposits in salt form being capable of depositing on the heat-accumulator material. Such typical salts are, in particular, ammonium hydrogen sulphite, ammonium sulphite, ammonium hydrogen sulphate and ammonium sulphate.
It is the object of the invention to provide a SCR process and a SCR system for high flow rates with low expenditure for equipment, low operating costs and high denitrification efficiency.
According to the invention, this is achieved by the process characterized in claim 1 and the system characterized in claim 9. Advantageous embodiments of the invention are described in the sub-claims.
In the process according to the invention for the purification of waste gas charged with nitrogen oxides in a reactor with heat-accumulator chambers containing heat-accumulator materials, which are also referred to as regenerators, the waste gas or raw gas to be purified alternately enters at least one of the heat-accumulator chambers. By means of the heat-accumulator material in this chamber, the raw gas is preheated, then mixed with a reducing agent for reducing the nitrogen oxides and supplied to a catalyst for reducing the nitrogen oxides. The denitrified hot clean gas then heats the heat-accumulator material in the at least one heat-accumulator chamber which it exits.
By means of a recirculation system, a partial flow is taken from the at least one heat-accumulator chamber which the clean gas exits before the clean gas enters the heat-accumulator material thereof and, heated by means of a heat source and mixed with the reducing agent, is supplied to the heat-accumulator chamber which the raw gas enters after the raw gas has exited the heat-accumulator material thereof.
This means the system according to the invention has 2n pairs of heat-accumulator chambers. For forming the pairs of heat-accumulator chambers, the reactor consists of two structurally separated heat exchanger materials in order to form the individual heat-accumulator chambers.
The number n may be 1, 2, 3 or another integer greater than 0. The n pairs of heat-accumulator chambers are connected to a raw gas channel, and n pairs of heat-accumulator chambers are connected to a clean gas channel. In the simplest case, the system has thus one pair of heat-accumulator chambers, with one heat-accumulator chamber being connected to the raw gas channel and one heat-accumulator chamber being connected to the clean gas channel. If, for example, the system has four heat-accumulator chambers, hence n equals 2, two heat-accumulator chambers are connected to the raw gas channel, and two heat-accumulator chambers are connected to the clean gas channel. After a period of preferably 1 to 3 minutes, the heat-accumulator chamber connected to the raw gas is connected to the clean gas, and the heat-accumulator chamber connected to the clean gas is connected to the raw gas. This switching takes place by means of two valves for each chamber, with the valves preferably being configured in the form of poppet valves. Due to this switching, the raw gas is provided with the heat which has previously been accumulated in the clean gas chamber, and vice versa.
Since in the system according to the invention the reduction catalyst is arranged between the heat-accumulator chambers above the heat-accumulator material, only low expenditure for equipment is required for the SCR reaction. In addition, as the denitrified hot clean gas is used for preheating the raw gas, high thermal efficiency of, for example, 90 to 95% and more can be achieved with the system according to the invention.
The heat source in the recirculation system may, for example, be a burner or a heat exchanger.
The reducing agent with which the heated clean gas is mixed in the recirculation system may be a nitrogen-hydrogen compound, thus, for example, ammonia, which can be used both in gaseous form and in an aqueous solution, or, for example, urea as an aqueous solution or, preferably, gaseous ammonia catalytically produced from a pre-evaporated urea solution, e. g. a urea hydrolysis catalyst or H-cat (cf. e. g. EP 2 172 266 A1). The latter has the advantage that 2 moles of ammonia are generated per mole of urea, which can react with NOx, whereas when urea is used directly, only one of the nitrogens contained in the molecule reacts with NOx and the second one generates N2O, which is a potent greenhouse gas.
In order to evenly introduce the reducing agent into the heat-accumulator chamber over the entire cross section and to uniformly supply the heat required for continuous operation to the preheated raw gas flow, a distribution grate is provided on the side of the heat-accumulator material of the heat-accumulator chambers facing the reduction catalyst for reducing the nitrogen oxides at a distance from the reduction catalyst, but in proximity to the heat-accumulator material.
By means of the distribution grate in the heat-accumulator chamber which the clean gas exits, a partial flow of the hot clean gas is taken via the recirculation system, which is further heated and, mixed with the reducing agent, supplied to the distribution grate in the heat-accumulator chamber in which the raw gas to be purified is preheated.
Preferably, the temperature of the partial flow heated by means of the heat source and mixed with the reducing agent is 270 to 550° C.
The size of the partial flow and the temperature thereof is calculated in such a way that the raw gas is heated to a temperature of 200 to 400° C., in particular 250 to 350° C., which is required for the SCR reaction.
Preferably, a bypass pipe for bypassing the heat source is provided in the recirculation system in order to set the temperature of the clean gas mixed with the reducing agent, which has been supplied to the distribution grate in the heat-accumulator chamber for preheating the raw gas to the temperature required for the SCR reaction. Thus, for example, a gas burner having a small installation size may be used as a heat source.
The heat-accumulator material preferably consists of extruded prism-shaped heat-accumulator bodies having a plurality of gas passage channels running in the direction of gas flow. Such heat-accumulator materials are described in EP 0 472 605 A1. The catalyst material may likewise consist of such extruded ceramic prism-shaped bodies with gas passage channels running in the direction of gas flow.
The catalyst may be made from the aforementioned SCR catalyst materials. The reduction catalyst may, for example, consist of TiO2 as the main component with 1 to 5 percent by weight of vanadium oxide and/or tungsten oxide. The SCR catalyst, thus the catalyst for the reduction of the nitrogen oxides, can be configured in such a way that the system according to the invention can also be used for the catalytic oxidation of dioxin and/or furan. The addition of a reducing agent is, of course, not provided if no denitrification but only oxidation of dioxin and/or furan takes place.
One problem with SCR systems is the so-called ammonia slip, i. e. traces of ammonia which cannot be converted by the catalyst and can thus escape into the open air. According to legal provisions, the waste gas released may only contain minor traces of ammonia, typically less than 20 ppm.
Therefore, in order to ensure a high mixing quality of the reducing agent with the raw gas, it is, according to the invention, not injected directly but in a partial flow of clean gas taken downstream of the SCR catalyst via the non-required distribution grate. The high quality of the mixture of the reducing agent with the partial flow of clean gas is ensured by the arrangement of the reducing agent nozzles, the place of insertion and, for example, by a static mixer downstream of the injection site.
In order to rule out an ammonia slip, the reducing agent is preferably added sub-stoichiometrically, i. e., for example, the NH3/NOx mole ratio is preferably less than 1.
In order to counteract the slip of the reducing agent during switching, the injection of the reducing agent in the recirculation system is interrupted shortly before switching, and the reducing agent residues in the partial flow of the recirculation pipe are expelled by the preheated clean gas. This ensures that no reducing agent is left in the distribution grate and upstream of the SCR catalyst at the time of switching. This will hardly affect the NOx purification, because, due to its accumulation capability, the SCR catalyst has accumulated sufficient reducing agent in order to maintain the DeNOx reaction.
The heat-accumulator material preferably consists of ceramic heat-accumulator bodies with prism-shaped channels and with an open porosity of less than 10%, in particular less than 5%, in order to avoid that NOx and/or the reducing agent or other pollutants can adsorbe on the surface of the heat exchanger.
The SCR catalyst material preferably consists of honeycomb blocks having channels running in the direction of flow.
In addition, an oxidation catalyst may be arranged directly on the heat-accumulator material. In this way, an additional supporting grid is saved. The oxidation catalyst serves the purpose of purifying oxidizable pollutants in the raw gas, e. g. organic compounds or carbon monoxide. This arrangement prevents the reducing agent from being also oxidized by the oxidation catalyst, because it is located upstream of the injection sites of the clean gas/reducing agent mixture. Since a further oxidation catalyst layer is arranged on the heat-accumulator material downstream of the SCR catalyst, it is ensured that pollutants which have not been fully oxidized as well as a possibly existing slip of the reducing agent are oxidized almost completely.